The present invention, in some embodiments thereof, relates to magnetic resonance analysis and, more particularly, but not exclusively, to magnetic resonance analysis using bipolar gradient pulse subsequences.
Magnetic resonance (MR) analysis is a technique for obtaining the chemical and physical microscopic properties of materials, by utilizing a quantum mechanical phenomenon, named Nuclear Magnetic Resonance (NMR), in which a system of spins, placed in a magnetic field resonantly absorb energy, when applied with a certain frequency.
A nucleus can experience NMR only if its nuclear spin I does not vanish, i.e., the nucleus has at least one unpaired nucleon. Examples of non-zero spin nuclei frequently used in MRI include 1H (I=1/2), 2H (I=1), 23Na (I=3/2), etc. When placed in a magnetic field, a nucleus having a spin I is allowed to be in a discrete set of energy levels, the number of which is determined by I, and the separation of which is determined by the gyromagnetic ratio of the nucleus and by the magnetic field. Under the influence of a small perturbation, manifested as a radiofrequency magnetic field, which rotates about the direction of a primary static magnetic field, the nucleus has a time dependent probability to experience a transition from one energy level to another. With a specific frequency of the rotating magnetic field, the transition probability may reach the value of unity. Hence at certain times, a transition is forced on the nucleus, even though the rotating magnetic field may be of small magnitude relative to the primary magnetic field. For an ensemble of spin I nuclei the transitions are realized through a change in the overall magnetization.
Once a change in the magnetization occurs, a system of spins tends to restore its magnetization longitudinal equilibrium value, by the thermodynamic principle of minimal energy. The time constant which control the elapsed time for the system to return to the equilibrium value is called “spin-lattice relaxation time” or “longitudinal relaxation time” and is denoted T1. An additional time constant, T2 (≦T1), called “spin-spin relaxation time” or “transverse relaxation time”, controls the elapsed time in which the transverse magnetization diminishes, by the principle of maximal entropy. However, inter-molecule interactions and local variations in the value of the static magnetic field, alter the value of T2, to an actual value denoted T2*.
In MR analysis, pulse sequences are applied to the object to generate NMR signals and obtain information therefrom which is subsequently used to analyze the object. The above mentioned relaxation times and the density distribution of the nuclear spin are properties which vary from one object to the other, and therefore allows the analysis of the object. In diffusion-weighted MR analysis, for example, magnetic field gradients are applied so as to provide motion-related contrast which is sensitive to motion of fluid molecules in selected directions. Diffusion-weighted MR analysis exploits the random motion of the molecules which causes a phase dispersion of the spins with a resultant signal loss. Such analysis can be used for characterizing morphological features of pores that are embedded within porous media, wherein molecules are diffusing within the pores in restricted manner.
A known technique for observing diffusion in porous media employs the so-called pulsed field gradient (PFG) sequence, wherein a pair of magnetic field gradient pulses is applied to encode displacements between the application of these two pulses.
U.S. Pat. No. 7,053,611, for example, discloses a method that includes acquiring a suite of NMR measurements of a fluid sample using a single-polar PFG (s-PFG) sequence for encoding diffusion information, wherein each NMR measurement in the suite is acquired with a different value in a parameter in the pulsed field gradient pulses for producing a different diffusion effect. The suite of NMR measurements is inverted to produce a distribution function that relates diffusion properties of the fluid sample with the longitudinal and/or transverse magnetic relaxation time thereof.
Bipolar PFG sequences have been used in medical imaging applications for measuring diffusion in heterogeneous laboratory samples in which the applied magnetic field is very homogeneous, but produces internal gradients because of the nature of the material of the sample. Additionally, bipolar PFG sequences have been used for measuring diffusion and relaxation of reservoir fluids in the pore spaces of earth formations surrounding a borehole (U.S. Pat. No. 5,796,252).
U.S. Published Application No. 20100033182 teaches a multi-PFG experiment, which involves the application of repeated pairs of diffusion gradients, and in particular a double PFG (d-PFG) sequence which includes two pairs of diffusion gradient pulses. An estimate of the size characteristic of a distribution of restricted compartments of the sample is generated based on the received MR signal.
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